Heat-stable polyimide resins

ABSTRACT

New heat-stable resins are provided having good mechanical and electrical properties combined with chemical inertness at temperatures of 200° to 300° C, which resins comprise three-dimensional recurring units of the general formula: ##STR1## in which, in any particular recurring unit: A IS ZERO OR AN INTEGER FROM 1 TO 5; ##STR2## denotes a radical of the general formula ##STR3##  p1 in which Y represents H, CH 3  or Cl; G denotes a monovalent aromatic radical; and 
     A denotes a divalent organic radical possessing at least two carbon atoms, 
     Such that there are on average, at least 4.4 and at most about ##STR4## radicals per ##STR5## radical, and optionally, units and/or molecules of the general formula ##STR6## IN WHICH: T denotes a monovalent organic radical; 
     T 1  denotes a radical of the general formula: ##STR7##  or a radical of the general formula: ##STR8## b is zero or an integer from 1 to 4; AND A, D 1  and G are as defined in claim 1; such that there are at most about 30 ##STR9## radicals per 100 ##STR10##  radicals.

This is a division of application Ser. No. 228,233 filed Feb. 22, 1972,now U.S. Pat. No. 3,998,787.

The present invention relates to new heat-stable resins based onthree-dimensional polyimides.

Three dimensional polyimides prepared by heating N,N-bis-imides ofunsaturated dicarboxylic acids (see French Pat. No. 1,455,514) or bycyclising-dehydration and crosslinking of polyamide-acids, the ends ofthe chains of which are blocked by an unsaturated dicarboxylic acid (seeFrench Pat. No. 1,537,135) have already been described.

Furthermore linear polymers blocked by maleimido groups, wherein therecurring unit has the formula: ##STR11## in which R represents aphenylene or hexamethylene radical, are known [see Stille and Anyos J.Polym. Sci. A. vol. 2, p. 1487 (1964)]. These polymers have beenobtained by heating the corresponding bis-maleimide with benzalazine,the reactants being used in stoichiometric amounts or with a molarexcess of bis-maleimide; the crude polymer is thereafter freed ofresidual monomers by washing with chloroform and acetone. The authorsalso indicate that the viscosity of the crude polymers decreases duringprolonged heating above 200° C. and regard this result as theconsequence of a depolymerisation taking place.

The present invention provides a new heat-stable resin which is based onthree-dimensional polyimides and which comprises three-dimensionalrecurring units of the general formula: ##STR12## in which, in anyparticular recurring unit: a is zero or an integer from 1 to 5;##STR13## denotes a radical of the general formula ##STR14## in which Yrepresents H, CH₃ or Cl; G denotes a monovalent aromatic radical; and

A denotes a divalent organic radical possessing at least two carbonatoms, such that there are on average, at least 4.4 and at most about##STR15## radicals per ##STR16## radical.

The resins of this invention can also contain units and/or molecules ofthe general formula: ##STR17## in which: T denotes a monovalent organicradical

T₁ denotes a radical ##STR18## or a radical ##STR19## b is zero or aninteger from 1 to 4; and A, D₁ and G are as defined above.

The resins which contain units and/or molecules of the formula (II)furthermore contain at most about 30 radicals ##STR20## per 100 radicals##STR21##

In the formulae given above, the symbol A can denote, for example, analkylene radical having less than 13 carbon atoms, a phenylene orcyclohexylene radical or one of the radicals of the formulae: ##STR22##wherein n represents an integer from 1 to 3, a divalent radical having12 to 30 carbon atoms consisting of phenylene or cyclohexylene radicalslinked to one another by a simple valency bond or by an inert atom orgroup such as --O-- or --S--, an alkylene group with 1 to 3 carbonatoms, --CO--, --SO₂ --, --NR₁ --, --N═N--, --CONH--, --COO--, --P(O)R₁--, --CONH--X--NHCO--, ##STR23## wherein R₁ represents a hydrogen atom,an alkyl radical with 1 to 4 carbon atoms, a phenyl radical or acyclohexyl radical and X represents an alkylene radical with less than13 carbon atoms. Furthermore, the various phenylene or cyclohexyleneradicals can be substituted by methyl groups.

The symbol G can represent for example, a phenyl radical optionallysubstituted by inert atoms, radicals or groups such as Cl, F, CH₃, OCH₃and NO₂.

The symbol T can represent for example, an alkyl radical with 1 to 18carbon atoms, a phenyl or cyclohexyl radical or ##STR24## wherein n isas defined above, or a monovalent radical consisting of a phenyl radicaland a phenylene radical linked to one another by a simple valency bondor by an inert atom or group such as --O--, --S--, an alkylene radicalwith 1 to 3 carbon atoms, --CO--, --SO₂ --, --NR₁ --, --N═N--, --CONH--or --COO--, wherein R₁ is as defined above. Furthermore, the ringsbelonging to these various radicals can be substituted by inert atoms,radicals or groups such as F, Cl, CH₃, OCH₃ and NO₂.

The resins consisting of units (I) can be prepared by heating, atbetween 50° C. and 350° C., a mixture comprising: an aldehyde-azine ofthe general formula:

    G -- CH ═ N -- N ═ CH -- G                         (III)

(hereafter referred to as an alazine), in which G is as defined above,and a bis-imide of the general formula: ##STR25## in which A and Y areas defined above, in such amounts that on average at least 2.2 and atmost 10 mols of bis-imide (IV) are present per mol of alazine.

The preparation of the resins which also contain units or molecules offormula (II) can be effected by heating, at between 50° C. and 350° C.,a mixture which in addition to containing the alazine of formula (III)and the bis-imide of formula (IV), in the proportions indicated above,contains a mono-imide of the general formula: ##STR26## in which Y and Tare as defined above, in such amounts that at most 30 mols of mono-imide(V) are present per 100 mols of bis-imide (IV).

It is to be understood that a mixture of alazines of formula (III) canbe used and that, mutatis mutandis, this remark equally applies to theimides of formulae (IV) and (V).

Amongst the alazines of formula (III) which can be used, there may bementioned benzalazine, p-methoxybenzalazine, p-nitrobenzalazine andp-chlorobenzalazine.

Specific examples of bis-imides of formula (IV) include:N,N'-ethylene-bis-maleimide, N,N'-hexamethylene-bis-maleimide,N,N'-metaphenylene-bis-maleimide, N,N'-paraphenylene-bis-maleimide,N,N'-4,4'-biphenylylene-bis-maleimide,N,N'-4,4'-diphenylmethane-bis-maleimide,N,N'-4,4'-diphenylether-bis-maleimide,N,N'-4,4'-diphenylsulphone-bis-maleimide,N,N'-4,4'-dicyclohexylmethane-bis-maleimide,N,N'-α,α'-4,4'-dimethylene-cyclohexane-bis-maleimide,N,N'-meta-xylylene-bis-maleimide, N,N'-para-xylylene-bis-maleimide,N,N'-4,4'-(1,1-diphenylcyclohexane)-bis-maleimide,N,N'-4,4'-diphenylmethane-bis-citraconimide,N,N'-4,4'-diphenylmethane-bis-chloromaleimide,N,N'-4,4'-(1,1-diphenylpropane)-bis-maleimide,N,N'-4,4'-(1,1,1-triphenylethane)-bis-maleimide,N,N'-4,4'-triphenylmethane-bis-maleimide andN,N'-3,5-(1,2,4-triazole)-bis-maleimide. These bisimides can be preparedaccording to the methods described in U.S. Pat. No. 3,018,290 andBritish Pat. Specification No. 1,137,592 for example.

Specific examples of mono-imides of formula (V) which can be usedinclude N-phenylmaleimide, N-phenylmethyl-maleimide,N-phenyl-chloromaleimide, N-p-chlorophenylmaleimide,N-p-methoxyphenyl-maleimide, N-p-methylphenylmaleimide,N-p-nitrophenylmaleimide, N-p-phenoxyphenyl-maleimide,N-p-phenylaminophenylmaleimide, N-p-phenoxycarbonylphenylmaleimide andN-p-phenylcarbonylphenylmaleimide. These monoimides can be prepared by,for example, the method described in U.S. Pat. No. 2,444,536 for thepreparation of N-arylmaleimides.

The preparation of the resins of this invention is advantageouslycarried out in two stages. In a first stage, a prepolymer (P) isprepared, which can then be shaped using it as a solution, a suspension,a powder or a liquid mass.

The prepolymers can be prepared in bulk by heating the mixture of thealazine, the bis-imide and, optionally, the monoimide, until ahomogeneous liquid is obtained; in the following description, thismixture will be referred to as "the mixture of the reactants".

The temperature at which the preparation of the prepolymer is carriedout can vary within rather wide limits depending on the nature andnumber of the reactants present, but it is generally between 80° and180° C. It is advantageous to homogenise the mixture of the reactantsbeforehand if the reactants have a relatively high melting point.

The prepolymers can also be prepared by heating the reactants in a polarsolvent such as dimethylformamide, N-methylpyrrolidone,dimethylacetamide, N-methylcaprolactam, diethylformamide andN-acetylpyrrolidone, at a temperature which is generally between 50° C.and 180° C. The solutions of prepolymers can be used as such fornumerous uses; it is also possible to isolate the prepolymer from itssolution by precipitation with a diluent which is miscible with thepolar solvent and does not dissolve the prepolymer, such as water or ahydrocarbon having a boiling point which does not significantly exceed120° C.

The prepolymers can be used as a liquid mass and it is sufficient toshape them by casting whilst hot. It is also possible, to cool and grindthem and then to use them in the form of powders which are remarkablysuitable for compression moulding processes, optionally in the presenceof fillers in the form of, for example, powders, spheres, granules,fibres or flakes. In the form of suspensions or solutions, theprepolymers can be used in the production of coatings and intermediatepreimpregnated articles, the reinforcement consisting of, for example,fibrous materials consisting of aluminium or zirconium silicate oraluminium or zirconium oxide, carbon, graphite or boron, asbestos orglass.

In a second stage, the prepolymers can be cured by heating totemperatures of the order of 350° C., generally between 150° and 300°C.; a supplementary shaping can be effected during curing, optionally invacuo or under super-atmospheric pressure; these operations can also beconsecutive. The curing can be effected in the presence of a radicalpolymerisation initiator such as lauroyl peroxide,azo-bis-isobutyronitrile or an anionic polymerisation catalyst such asdiazabicyclooctane.

The resins of this invention can also contain, as an adjuvant, anaromatic compound (AR) containing from 2 to 4 benzene rings, which isnot sublimable at atmospheric pressure up to 250° C. and has a boilingpoint above 250° C.; the addition of these aromatic compounds generallycauses a lowering in the softening point of the prepolymers. In thesearomatic compounds, the benzene rings can form condensed nuclei or canbe bonded to one another by a valency bond or by an inert atom or groupsuch as ##STR27## or a combination of these various types of linking canbe present (in a single compound). The benzene rings can be substitutedby inert radicals such as --CH₃, --OCH₃, --F, --Cl and --NO₂. Examplesinclude the isomeric terphenyls, the chlorinated diphenyls, phenylether, 2,2'-naphthyl ether, o-methoxylphenyl ether, benzophenone,2,5,4'-trimethylbenzophenone, p-phenylbenzophenone,p-fluorobenzophenone, diphenylamine, diphenylmethylamine,triphenylamine, azobenzene, 4,4'-dimethylazobenzene, azoxybenzene,diphenylmethane, 1,1-diphenylethane, 1,1-diphenylpropane,triphenylmethane, diphenylsulphone, phenyl sulphide, 1,2-diphenylethane,p-diphenoxybenzene, 1,1-diphenyl-phthalane, 1,1-diphenylcyclohexane,phenyl benzoate, benzyl benzoate, p-nitrophenyl terephthalate andbenzanilide. These aromatic adjuvants can be used in amounts of up toabout 10% by weight relative to the weight of the prepolymer (P) or tothe weight of the mixture of the reactants. According to an advantageousembodiment, the adjuvant (AR) is added to the prepolymer (P) orintroduced into the mixture at any time during its preparation.

The resins of this invention can also be modified by the addition,before curing, of a monomer (M) other than an imide and containing atleast one polymerisable --CH ═ C< group of the vinyl, maleic, allyl oracrylic type. The monomer can possess several --CH ═ C< groups providedthe double bonds are not conjugated. In one and the same monomer, it ispossible for more than one type to be present and it is also possible touse a mixture of copolymerisable monomers.

The monomers which can be used are generally esters, ethers,hydrocarbons, substituted heterocyclic derivatives, organo-metalliccompounds or organometalloid compounds.

Suitable esters include the vinyl, allyl, methallyl, 1-chloroallyl,crotyl, isopropenyl and cinnamyl esters derived from saturated orunsaturated aliphatic or aromatic mono- or poly-carboxylic acids, suchas formic, acetic, propionic, butyric, oxalic, malonic, succinic,adipic, sebacic, acrylic, methacrylic, phenylacrylic, crotonic, maleic,fumaric, itaconic, citraconic, tetrahydrophthalic,acetylene-dicarboxylic, benzoic, phenylacetic, orthophthalic,terephthalic and isophthalic acid, as well as the esters ofnon-polymerisable alcohols such as the methyl, isopropyl, 2-ethylhexyland benzyl esters derived from polymerisable acids such as thosementioned above. Typical examples are vinyl acetate, allyl acetate,methyl acrylate and methacrylate, vinyl methacrylate, allyl maleate,allyl fumarate, allyl phthalate and allyl malonate.

Suitable ethers include vinyl allyl ether, allyl ether, methallyl ether,allyl crotyl ether and vinyl phenyl ether.

Amongst the substituted heterocyclic compounds, there may be mentionedthe vinylpyridines, N-vinyl-pyrrolidone, N-vinylcarbazole, allylisocyanurate, vinyltetrahydrofurane, vinyldibenzofurane,allyloxytetrahydrofurane and N-allylcaprolactam.

Hydrocarbons such as styrene, alpha-methylstyrene, p-chlorostyrene,vinylcyclohexane, 4-vinylcyclohexene, divinylcyclohexane, diallylbenzeneand vinyltoluene can be used.

Suitable monomeric organometallic and organometalloid derivativesinclude those containing one or more atoms of phosphorus, boron orsilicon. These can be silanes or siloxanes, phosphines, phosphine oxidesor sulphides, phosphates, phosphites, phosphonates, boranes,orthoborates, boronates, boroxoles, borazoles and phosphazenes. Examplesinclude vinyloxytrimethylsilane, 1,3-diallyl-tetramethyldisiloxane,allyldimethylphosphine oxide, allyl orthophosphate, allylmethylphosphonate, methyl paravinylphenylboronate, triallylborazole,triallylboroxole, triallytrichlorophosphazene, allyl phosphate and allylallylphosphonate.

Furthermore, the monomers referred to above can contain halogen atoms,principally chlorine or fluorine, or functional groups such as analcoholic or phenolic hydroxyl group, an aldehyde carbonyl group or anamido, epoxy or nitrile group. Examples of suitable monomers (M) whichcontain such substituents include allyloxyethanol, p-allyloxyphenol,tetraallylepoxyethane, glycidyl acrylate, glycidyl methacrylate, allylglycidyl ether, 4-vinyl-epoxycyclohexane, p-cyanostyrene, acrylamide,N-methylacrylamide, N-allylacrylamide, N-methylolacrylamide, methylvinyl ketone, methyl allyl ketone, acrylonitrile, methylacrylonitrile,p-chlorostyrene and p-fluorostyrene.

The monomer (M) can be added to the prepolymer (P) or introduced intothe mixture at any time during the preparation. The amount used is lessthan 50%, preferably 5 to 40%, of the weight of the prepolymer (P) or ofthe weight of the mixture of the reactants. The curing of the prepolymermodified with the monomer (M) can be effected under similar conditionsto those which may be used for the curing of the unmodified prepolymer.

The resins of this invention can also be modified by the addition of anunsaturated polyester before curing. The unsaturated polyesters whichcan be used are well-known products. They are usually prepared bypolycondensation of polycarboxylic derivatives and polyols, at least onecontaining olefinic unsaturation; by polycarboxylic derivatives thereare meant acids, esters of lower alcohols, acid chlorides and, whereappropriate, acid anhydrides. Preferred unsaturated polyesters are thosederived from diacids or dianhydrides containing an olefinic double bondin the α,β-position. By way of example, the polycarboxylic derivativescan be of the maleic, chloromaleic, itaconic citraconic, aconitic,pyrocinchonic, fumaric, chlorendic, endomethylene-tetrahydrophthalic,tetrahydrophthalic, ethylmaleic, succinic, sebacic, phthalic,isophthalic, adipic and hexahydrophthalic type. Amongst the polyols, themost commonly used are ethylene glycol, propylene glycol, diethyleneglycol, triethylene glycol, neopentylglycol, tetraethylene glycol,butylene glycol, dipropylene glycol, glycerol, trimethylpropane,pentaerythritol, sorbitol and 3,3-bis(hydroxymethyl)-cyclohexene.

The term "unsaturated polyester" also covers solutions of thepolycondensates described above in a monomer (M') which can copolymerisewith them. These monomers are also well known in polyester technology;typical examples include styrene, alpha-methylstyrene, vinyltoluene,p-(alpha-methylvinyl)benzophenone, divinylbenzene, vinyl 2-chloroethylether, N-vinylpyrrolidone, 2-vinylpyridine, indene, methyl acrylate,methyl methacrylate, acrylamide, N-t-butylacrylamide, acrylonitrile,hexahydro-1,3,5-triacrylo-s-triazine, allyl phthalate, allyl fumarate,allyl cyanurate, allyl phosphate, diethylene glycol diallyl carbonate,allyl lactate, allyl malonate, allyl tricarballylate, allyl trimesateand allyl trimellate, If a monomer (M') is used, it generally representsfrom 10% to 60% of the weight of the solution of unsaturated polyester.

The unsaturated polyesters can be prepared in known manner; on thissubject, reference may, for example, be made to KIRK-OTHMER:Encyclopaedia of Chemical Technology, 2nd edition, volume 20.

The details relating to the introduction and amounts of unsaturatedpolyester as well as to the curing to give resins are identical to thosewhich have been described above in connection with monomer (M).

The incorporation of a monomer (M) or of an unsaturated polyester givescurable mixtures which can be used as impregnating resins; afteraddition of fillers, they can be used as coating compositions.

The resins according to the invention are of value in industries whichrequire materials possessing good mechanical and electrical propertiesas well as high chemical inertness at temperatures of 200° C. to 300° C.By way of example, they are suitable for the manufacture of insulatingmaterials, in sheet or tube form, for electrical transformers, printedcircuits, and self-lubricating gears, collars and supports.

The following Examples further illustrate the present invention.

EXAMPLE 1

107.3 g. of N,N'-4,4'-diphenylmethane-bis-maleimide and 20.83 g. ofbenzalazine are intimately mixed. The mixture is thereafter spread on aheated plate and kept in a heated chamber at 150° C. for 29 minutes.After cooling, the prepolymer is finely ground (particle diameter lessthan 100μ). A powder of softening point about 130° C. is obtained.

25 g. of this powder are introduced into a cylindrical mould (diameter:76 mm) which is placed between the platens of press which has beforehandbeen heated to 250° C. The whole is kept at this temperature for 1 hourunder a pressure of 250 bars.

After release from the mould whilst hot, the article is subjected to asupplementary heat treatment at 250° C. for 24 hours. After cooling, ithas a flexural breaking strength of 10.3 kg/mm² at 25° C. After a heattest lasting 1,800 hours at 250° C., this strength is still 9.8 kg/mm².

EXAMPLE 2

83.25 g. of benzalazine are dissolved in 294 g. of dimethylformamide andthe soluton is then heated to 130° C. 358 g. ofN,N'-4,4'-diphenylmethane-bis-maleimide are then introduced and thewhole is kept at 130° C. for 2 hours 30 minutes. After cooling, thesolution is introduced, over 5 minutes, into 1.2 l. of vigorouslystirred water; the prepolymer precipitates and it is washed four timeswith 250 cm³ of water at 70° C. After drying at 60°/3mm Hg, 429 g. of apowder of softening point about 152° C. are obtained.

25 g. of this powder are taken and moulded under the conditionsdescribed in Example 1. The moulded article has a flexural breakingstrength of 12 kg/mm² at 25° C. After a heat test lasting 840 hours at250° C. this strength is still 9.6 kg/mm².

EXAMPLE 3

108 g. of N,N'-4,4'-diphenyl-ether-bis-maleimide and 20.83 g ofbenzalazine are intimately mixed. The mixture is spread on a plate andthe whole is kept at 160° C. for 31 minutes and then at 200° C. for 30minutes. After cooling, the prepolymer is finely ground and 0.43 g. ofdiazabicyclooctane are then added to it.

12.5 g of this mixture are taken and 12.5 g. of short glass fibres (3mm. length) are incorporated into it. Moulding is then carried out withthis filled prepolymer under the conditions described in Example 1. Themoulded article has a flexural breaking strength of 27.8 kg/mm² at 25°C. At 250° C., this strength is 18.3 kg/mm².

EXAMPLE 4

The experiment described in the preceding Example is repeated but using80.4 g. of N,N'-4,4'-metaphenylene-bis-maleimide as the bis-imide, theprepolymer being obtained by heating the mixture at 160° C. for 31minutes.

The moulded article has a flexural breaking strength of 21.7 kg/mm² at25° C. At 250° C., this strength is 19.5 kg/mm².

EXAMPLE 5

A prepolymer is prepared by heating an intimate mixture of 214.6 g. ofN,N'-4,4'-diphenylmethane-bis-maleimide and 47.5 g. of benzalazine at150° C. for 19 minutes. The prepolymer is thereafter finely ground. 28g. of the powder are taken and introduced into 34 g. of water, withvigorous stirring.

13.5 dm² of a satin type of glass fibre fabric, having a specific weightof 308 g/m², are impregnated with the suspension thus prepared; thisfabric has previously been desized by heating and then treated withγ-aminopropyltriethoxysilane. The coated fabric is thereafter dried at130° C. for 25 minutes at 500 mm/Hg. 12 square samples (10 cm × 10 cm)are cut from this pre-impregnated fabric and are stacked so as toalternate the weft and warp. The stack is thereafter placed between theplatens of a press pre-heated to 250° C. A pressure of 40 bars isapplied and the whole is then kept under these conditions for 30minutes. During cooling, the laminate is released when the temperaturereaches 150° C. This laminate contains 35.8% by weight of resin and hasa flexural breaking strength of 38.6 kg/mm² at 250° C. After a heat testlasting 500 hours at 250° C., this strength is 39.4 kg/mm² (againmeasured at 250° C.).

EXAMPLE 6

268.7 g of N,N'-4,4'-diphenylmethane-bis-maleimide and 52 g ofbenzalazine are intimately mixed. Three 63 g. samples are taken fromthis mixture and three compositions C₁, C₂ and C₃ are prepared therefromby respectively adding to them:

7 g of terphenyl (C₁)

7 g of allyl phthalate (C₂)

7 g of an unsaturated polyester prepared by heating

46 kg of chlorendic acid, 13.7 g of maleic acid and

14.85 kg of ethylene glycol under nitrogen at 175° C. for 12 hours (C₃).

Each of the compositions is heated to 150° C. for 41 minutes; aftercooling, compositions C₁ and C₂ are subjected to a supplementaryheating, rising from 85° to 110° C., for 15 hours. After grinding,powders are obtained which are moulded under the conditions described inExample 1. The moulded articles respectively have a flexural breakingstrength of 13, 11.4 and 13.5 kg/mm² at 25° C. After a heat exposure at250° C., this strength assumes the following values:

9.1 kg/mm² for (C₁) after 1,000 hours.

7.9 kg/mm² for (C₂) after 1,490 hours.

8.5 kg/mm² for (C₃) after 1,000 hours.

EXAMPLE 7

The experiment described in Example 1 is repeated, but starting from35.8 kg of N,N'-4,4'-diphenylmethane-bis-malemide and 9.46 g ofbenzalazine; furthermore, moulding is carried out under 200 bars. Thesupplementary heat treatment is carried out at 250° C. for 48 hours.After this treatment, the material has a flexural breaking strength of9.9 kg/mm² at 25° C.; at 250° C. this strength is 5.5 kg/mm².

EXAMPLE 8

A prepolymer is prepared from a mixture which in addition to the twoessential reactants used in the preceding Example contains 4 g ofN-phenylmaleimide; the mixture is heated at 150° C. for 25 minutes.After cooling, moulding and a supplementary heat treatment are carriedout under the conditions described in Example 7. After a heat testlasting 170 minutes at 250° C., the loss in weight observed in a sampleof the moulded article is less than 2.4%.

I claim:
 1. A heat-stable cured resin which consists essentially ofthree-dimensional recurring units of the general formula: ##STR28## inwhich, in any particular recurring unit a is zero or an integer from 1to 5; ##STR29## denotes a radical of the general formula: ##STR30## inwhich Y represents H, CH₃ or Cl; G denotes a monovalent carbocyclicaromatic radical; andA denotes an alkylene radical having less than 13carbon atoms, a phenylene or cyclohexylene radical or one of theradicals of the formulae: ##STR31## wherein n represents an integer from1 to 3, or a divalent radical having 12 to 30 carbon atoms consisting ofphenylene or cyclohexylene radicals linked to one another by a simplevalency bond or by --O-- or --S--, and alkylene group with 1 to 3 carbonatoms, --CO--, --SO₂ --, --NR₁ --, --N═N--, --CONH--, --COO--, --P(O)R₁--, --CONH--X--NHCO--, ##STR32## wherein R₁ represents a hydrogen atom,an alkyl radical with 1 to 4 carbon atoms, a phenyl radical or acyclohexyl radical and X represents an alkylene radical with less than13 carbon atoms. such that there are, on average, at least 4.4 and atmost about 20 >D₁ < radicals per ##STR33## radical which is modified bythe addition, before curing of the resin, of a polymerisable monomerother than an imide, containing at least one vinyl, maleic, allyl oracrylic --CH═C< group, in C<amount from 5% to 50% by weight based on theweight of the reactants.
 2. The resin according to claim 1 which alsocontains units and/or molecules of the general formula: ##STR34## inwhich: T denotes a monovalent organic radical;T₁ denotes a radical ofthe general formula: ##STR35## or a radical of the general formula:##STR36## b is zero or an integer from 1 to 4; and A, D₁ and G are asdefined in claim 1; such that there are at most about 30 ##STR37##radicals per 100 ##STR38## radicals.
 3. The resin according to claim 1in which A denotes an alkylene radical having less than 13 carbon atoms,a phenylene or cyclohexylene radical or one of the radicals of theformulae: ##STR39## wherein n represents an integer from 1 to 3, adivalent radical having 12 to 30 carbon atoms consisting of phenylene orcyclohexylene radicals linked to one another by a simple valency bond orby an inert atom or group --O-- or --S--, an alkylene group with 1 to 3carbon atoms, --CO--, --SO₂ --, NR₁ --, --N═N--, --CONH--, --COO--,--P(O)R₁ --, --CONH--X--NHCO--, ##STR40## wherein R₁ represents ahydrogen atom, an alkyl radical with 1 to 4 carbon atoms, a phenylradical or a cyclohexyl radical and X represents an alkylene radicalwith less than 13 carbon atoms.
 4. The resin according to claim 3 inwhich A denotes: ##STR41##
 5. The resin according to claim 1 in which Gdenotes a phenyl radical optionally substituted by a chlorine, fluorine,methyl, methoxy or nitro group.
 6. The resin according to claim 4 inwhich G denotes a phenyl radical.
 7. The resin according to claim 5 inwhich T denotes an alkyl radical to 1 to 18 carbon atoms, a phenyl orcyclohexyl radical or a ##STR42## radical wherein n represents aninteger from 1 to 3, or a phenyl and a phenylene radical connected by asingle bond or by --O--, --S--, an alkylene radical of 1 to 3 carbonatoms, --CO--, --SO₂ --, --NR₁ --, --N═N--, --CONH-- or --COO-- whereinR₁ is as defined in claim
 3. 8. The resin according to claim 7 in whichT denotes a phenyl radical.
 9. The resin according to claim 1 which alsocomprises an aromatic compound containing 2 to 4 benzene rings which isnot sublimable at atmospheric pressure up to 250° C. and which has aboiling point above 250° C.
 10. The resin according to claim 9 whichalso comprises a terphenyl.
 11. The resin according to claim 1 in whichthe polymerisable monomer is allyl phthalate.
 12. Process for thepreparation of a resin as defined in claim 1 which comprises heating atbetween 50° and 350° C., at least one alazine of the general formula:

    G -- CH ═ N -- N ═ CH -- G

in which the symbol G is as defined in claim 1 with at least onebis-imide of the general formula: ##STR43## in which A and Y are asdefined in claim 1, in such amounts that on average at least 2.2 and atmost 10 mols of bis-imide are present per mol of alazine, and apolymerisable monomer as defined in claim
 1. 13. Process according toclaim 12 in which a mono-imide of the general formula: ##STR44## inwhich G, A, Y are as defined in claim 12 and T is as defined in claim 2is heated with the alazine and bis-imide in an amount such that at most30 mols of mono-imide are present per 100 mols of bis-imide.
 14. Processaccording to claim 12 in which a prepolymer is first prepared by heatingthe mixture in bulk at a temperature of between 80° and 180° C. until ahomogeneous liquid is obtained and the prepolymer is then cured bysubsequent heating at between 150° and 350° C.
 15. Process according toclaim 12 in which a prepolymer is prepared by heating the reactants insolution in a polar solvent at a temperature of between 50° and 180° C.and the prepolymer is then cured by subsequent heating at between 150°and 300° C.
 16. Process according to claim 12 in which the polymerisablemonomer is allyl phthalate.
 17. A shapable heat curable prepolymerobtained by carrying out the first step in the process claimed in claim14.
 18. A shapable heat curable prepolymer solution obtained by carryingout the first step in the process claimed in claim 15.